The Art of Assembly Language, 2nd Edition

4.32 Anonymous Unions

Within a record declaration you can place a union declaration without specifying a fieldname for the union object. The following example demonstrates the syntax for this:

type
     HasAnonUnion:
          record
               r:real64;
               union
                    u:uns32;
                    i:int32;
               endunion;
               s:string;
          endrecord;

static
     v: HasAnonUnion;

Whenever an anonymous union appears within a record you can access the fields of the union as though they were direct fields of the record. In the example above, for example, you would access v's u and i fields using the syntax v.u and v.i, respectively. The u and i fields have the same offset in the record (8, because they follow a real64 object). The fields of v have the following offsets from v's base address:

v.r           0
     v.u           8
     v.i           8
     v.s          12

@size(v) ...